<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <style>
        * {
            margin: 0;
            padding: 0;
            box-sizing: border-box;
        }

        .content {
            overflow: hidden;
            width: 1000px;
            height: 3000px;
            background-color: pink;
            margin: 0 auto;
        }

        .backtop {
            display: none;
            width: 50px;
            left: 50%;
            margin: 0 0 0 505px;
            position: fixed;
            bottom: 60px;
            z-index: 100;
        }

        .backtop a {
            height: 50px;
            width: 50px;
            background: url(./images/bg2.png) 0 -600px no-repeat;
            opacity: 0.35;
            overflow: hidden;
            display: block;
            text-indent: -999em;
            cursor: pointer;
        }

        .header {
            position: fixed;
            top: -80px;
            left: 0;
            width: 100%;
            height: 80px;
            background-color: purple;
            text-align: center;
            color: #fff;
            line-height: 80px;
            font-size: 30px;
            transition: all .3s;
        }

        .ms {
            width: 300px;
            height: 300px;
            background-color: skyblue;
            margin-top: 600px;
        }
       
    </style>
</head>
<body>
    <div class="header">我是top导航栏元素</div>
    <div class="content">
        <div class="ms">秒杀模块我是子级元素</div>
    </div>
    <div class="backtop">
        我是div父级元素
        <img src="/study-js/作业/王者荣耀/images/1.webp" alt="">
        <a href="#">父级</a>
    </div>
    <script>
        let header = document.querySelector('.header');
        let ms = document.querySelector('.ms');
        window.addEventListener('scroll', function () {
            if (document.documentElement.scrollTop >= ms.offsetTop) {
                header.style.top = 0;
            } else {
                header.style.top = '-80px';
            }
            backtop.style.display = n >=ms.offsetTop? 'block':'none'
            backtop.addEventListener('click',function(){
                document.documentElement.scrollTop = 0
            })
        });
    </script>
</body>
</html>